The present invention relates to the aircraft indicator art and, in particular to a non-normalized angle-of-attack indicating system.
Angle-of-attack refers to the angle between an aircraft's wing and the oncoming air. For maneuvering flight, or for flight without reference to air speed, angle-of-attack is an extremely useful measure of the aerodynamic state of an aircraft's wing.
FIG. 1a depicts angles of attack for a typical commercial transport corresponding to the following states of the wing, states which are of particular interest to the pilot:
C.sub.Lmax, (the maximum lift coefficient developed by the wing); stick-shaker (the predetermined angle-of-attack at which the cockpit stick-shaker is activated to discourage the flight crew from approaching a stall); approach reference (the angle-of-attack to be flown during the approach); and zero lift (the angle-of-attack at which the aircraft's wing develops zero lift). Note that the values vary significantly with changes in aircraft configuration such as flap position.
Figure 1b depicts the relationship between two of the aforementioned angles of attack and Mach number. Notice that there is a significant variation in these angles over the range of Mach numbers experienced by a typical commercial transport aircraft.
Prior to the present invention, angle-of-attack information has been displayed using a so-called normalized display, in which two key angles of attack (for example, stick-shaker and zero lift) are used as anchor points on the scale. For example, stick-shaker angle-of-attack (.alpha..sub.ss) might be shown at the 12o'clock position on the display and zero-lift angle-of-attack (.alpha..sub.zl) at the 6o'clock position as shown in FIG. 2a. Then, the angle at which to display the pointer, b (measured in degrees, counterclockwise from the 6 o'clock position) would be: ##EQU1##
Using Equation 1, an angle-of-attack halfway between .alpha..sub.ss and .alpha..sub.zl would be shown at 90.degree., i.e. at the 3o'clock position.
The above-described method of depicting angle-of-attack has a drawback. As can be seen from FIG. 1b, for some aircraft the angle-of-attack at stick shaker varies significantly with Mach number. For the aircraft as shown, it drops from 11.degree. angle-of-attack at low Mach to about 6.degree. angle-of-attack at Mach numbers typical of cruising flight. If the air data measurement is corrupted for some reason (perhaps by a sensor or avionics failure) such that Mach number is estimated incorrectly, then .alpha..sub.ss and .alpha..sub.zl will be incorrectly estimated, and the scaling defined by Equation 1 will change the needle angle, causing the indicator to be misleading.
For example, consider the aircraft of FIGS. 1a and 1b flying at a low Mach number (.alpha..sub.ss =11.degree. angle-of-attack, .alpha..sub.zl =-2.degree. angle-of-attack) and at 4.5.degree. angle-of-attack. According to Equation 1, the pointer should be displayed at the 3o'clock position (i.e. the midpoint of the scale, or 90.degree. towards "SS" from "ZL") as shown in FIG. 2a. However, if the Mach number is erroneously computed to be in the cruise range (say Mach 0.75) then using Equation 1 with .alpha..sub.ss =6.degree. angle-of-attack produces an incorrect pointer angle of 145.degree. as shown in FIG. 2b. This is clearly not a desirable state of affairs during such an air data failure, when the angle-of-attack indicator in needed the most.
The same argument above applies to any re-scaling of the display based on aircraft configuration parameters such as flap position, which would further render the traditional normalized display useless if configuration sensing were inoperative or erroneous.